Additional supplies, troops, or support sent to help someone or strengthen something that already exists.
From 're-' (again) plus 'enforce' (from Old French 'enforcer'), literally meaning 'to make strong again.' Military usage became dominant, but the word applies broadly to any strengthening.
The prefix 're-' in 'reinforcements' is telling—it means you're making something strong again, suggesting that whatever it is had weakened. In military history, the call for reinforcements often meant the difference between victory and defeat.
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